Vertebral implant

ABSTRACT

An implant for insertion between a pair of spaced vertebrae has two interfitted parts relatively shiftable along an axis and having outer ends turned axially away from each other and each formed with an axially outwardly directed part-spherical convex surface. Formations such as a screwthread engaged between the parts can axially shift them relative to each other and lock their axial positions relative to each other. Respective L-shaped mounting brackets each have one leg extending transversely across the axis and formed with a part-spherical concave seat complementary to and fitting with a respective one of the part outer ends and another leg extending generally axially outward. The brackets are adapted to be fitted to the vertebrae. Fasteners secure each of the other legs to the respective vertebra.

FIELD OF THE INVENTION

The present invention relates to a vertebral implant. More particularlythis invention concerns such an implant for replacing a vertebra of thespinal column.

BACKGROUND OF THE INVENTION

When a vertebra is broken or crushed it is frequently necessary toablate the body of the crushed or broken vertebra or vertebrae, normallyalong with the flanking disks. In order, however, to prevent the spinalcolumn from collapsing with damage to the fragile spinal cord running inthe vertebral foramen forward of the vertebral body, it is necessary toemploy an implanted spacer. This device is braced vertically between thebodies of the adjacent vertebra and holds them apart at the desiredspacing. It may even serve to distract two vertebrae which have becometoo closely spaced due to crushing of a vertebra or disk.

To this end as described in US 2004/0049271, US 2005/0113921, U.S. Pat.No. 6,015,436, and CA 2,216,450 the implant has two parts that can bemoved and locked relative to each other along an axis extending betweenthe two disks. This way the implant can be shortened and fitted in placeby the surgeon, then extended to lock in place and even distract theadjacent vertebrae.

Frequently after insertion the implant is intended to become fused inplace, creating a region including itself and the two adjacent vertebrawhere the spine is completely rigid. This constitutes a disability thatis normally considered more desirable than the condition the implant wasinserted to treat.

It has been suggested, however, in US 2004/0210312 and 2005/0060036 toprovide some degree of movement in the implant. This eliminates thecomplete rigidity otherwise produced, but such implants are complex toinstall and not normally considered a safe treatment for an activeperson.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide animproved vertebral implant.

Another object is the provision of such an improved vertebral implantthat overcomes the above-given disadvantages, in particular that fullymaintains the distractibility of the two parts in order to preservemobility in the affected segments of the spinal column.

SUMMARY OF THE INVENTION

An implant for insertion between a pair of spaced vertebrae hasaccording to the invention two interfitted parts relatively shiftablealong an axis and having outer ends turned axially away from each otherand each formed with an axially outwardly directed part-spherical convexsurface. Formations such as a screwthread engaged between the parts canaxially shift them relative to each other and lock their axial positionsrelative to each other. Respective L-shaped mounting brackets each haveone leg extending transversely across the axis and formed with apart-spherical concave seat complementary to and fitting with arespective one of the part outer ends and another leg extendinggenerally axially outward. The brackets are adapted to be fitted to thevertebrae. Fasteners secure each of the other legs to the respectivevertebra.

Thus according to the invention the mounting brackets are each attachedto a respective one of the vertebrae and each have a curved bearing seatthat engages one of the parts having a correspondingly curved bearingsurface.

As a result of this design, first of all a mounting bracket is providedbetween the one part and the vertebra, which for achieving the desiredmobility allows a relative motion of the part with respect to themounting bracket. Hence it is not necessary to move the part itselfrelative to the vertebra and in direct contact with same. The mountingbracket may therefore be permanently and stably attached to thevertebra, and thus via the part allows force to be introduced over alarge area with a low surface pressure in order to avoid damaging thevertebra itself. This mobility provided in the spinal column is achievedby moving the part relative to the bearing seat, thereby allowing thedesired tilting motions due to the fact that the mutually contactingsurfaces of the bearing seat and part roll off one another as a resultof the curvature of these correspondingly shaped surfaces. The mobilitybetween the vertebrae adjoining the space may be achieved in principleby a bearing seat cooperating with a bearing surface. It is within thescope of the invention, and in fact is particularly preferred for twomounting brackets to be provided, one mounting bracket being attached toeach of the vertebrae, and for each of the parts to have a curvedbearing surface which corresponds to the associated bearing seat. Thisdesign achieves mobility at the edges of the space and prevents highbending stresses from occurring in the one vertebra or the adjacentpart.

Furthermore, within the scope of the invention the design is chosen sothat the bearing seat has a concave curvature and the bearing surfacehas a convex curvature, so that the two parts together with the twobearing seats result in a type of ball-and-socket joint which permitsmobility not only about a specified rotational axis, but also allowstilting motions as well as rotary motions of the spinal column.

When the radius of curvature of the bearing surface is the same for bothof the parts, translation between the two bearing seats, and thusincreased introduction of force into one of the bearing seats, isavoided.

To achieve a secure seat for the bearing shell on the vertebra, the longbracket leg formed with the bearing has on its side contacting thevertebra at least one tooth, it being preferred when the tooth has abeveled edge and is provided in a plurality. The teeth may be situatedon a circle so that a uniform introduction of force into the vertebra isachieved on the circumference of the circle. In addition, for securelyattaching the bearing shell to the vertebra the mounting bracket has atab on the side facing away from the bearing seat for laterallyoverlapping the vertebra. This tab or short leg sets is not only thelateral alignment of the mounting bracket with respect to the vehiclebody, but also offers the possibility for anchoring at least one bonescrew for fastening the short leg, which has at least one hole, to therespective vertebra.

As a result of the design according to the invention, in which therelative motion of the parts with respect to the vertebra is transmittedvia the mounting brackets, it is possible to design the two parts in theshape of a sleeve and to connect them to one another via an internalthread and an external thread, so that, although these parts undergorelative rotation with respect to the mounting bracket duringdistraction, this relative rotation is not critical due to the fact thattorque is not exerted directly on the vertebrae, and instead only theconvex bearing surface slides over the concave bearing seat. However, inorder to avoid this sliding motion during the distraction of the twoparts, it has proven satisfactory to select the design in such a waythat the two parts are sleeves, axially engage one inside the another,and are displaceably guided relative to one another in the axialdirection. A center part or ring having an internal thread is provided,and one of the sleeves makes a threaded connection with the center ringand is overlapped by both the center ring and the other sleeve. In thisembodiment, the rotary motion necessary for distraction of the implantis exerted by the center ring, the two parts being secured againsttwisting and being displaced relative to the center ring in the mannerof a spindle.

It is also advantageous for the sleeves to have radially throughgoingholes to allow the growth of tissue into the sleeve interior, or topermit active introduction of bone grafts or bone cement.

In addition, the center ring has key openings uniformly distributed overthe circumference which may be used for introducing a tool by thesurgeon and for twisting the center ring. To allow the length of theimplant to be permanently maintained and secured after distraction ofthe implant to the required length has been achieved, one of the sleeveshas a threaded hole in the overlap region with the other sleeve foraccommodating a locking screw.

It has proven to be advantageous for the parts and/or the mountingbrackets to be made of titanium, steel, or polyetheretherketone (PEEK),the selection of titanium or PEEK in particular making it possible to doimaging using nuclear magnetic resonance tomography without interferencefrom artifacts.

Making adjustments to the size of the vertebrae may be advantageous whenthe mounting bracket is changeable and fixable in its longitudinalextension, in particular when the mounting bracket has a subdivideddesign outside the bearing seat and has mutually adjustable components.The longitudinal extension may be easily changed by use of a telescopingdesign, or by a threaded rod which is seated in a threaded hole andwhose position is fixed by a locking screw.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a partly sectional side view of an implant inserted into thespace created by removal of a vertebra between the two remainingvertebrae;

FIG. 2 is a view like 1, the vertebrae shown in FIG. 1 being omitted forclarity;

FIG. 3 is a view like FIG. 2, with a deflection from the neutralposition; and

FIG. 4 is a view FIG. 2, with an opposite deflection from that shown inFIG. 3.

SPECIFIC DESCRIPTION

As seen in FIG. 1 an implant 1 is intended for insertion between twovertebrae 2 of the spinal column as a placeholder for a vertebra and/ordisks removed from the spinal column. This implant 1 according to theinvention may also be used when a vertebra 2 has not been entirelyremoved from the spinal column, but instead the facet joints 3, forexample, may have been retained in the spinal column. This implant 1comprises two parts 4′ and 4″ which may be displaced relative to oneanother in the direction of their coaxial longitudinal axis A forchanging the length of the implant 1. This is done by forming the twoparts 4′ and 4″ as interfitting and coaxial cylindrical sleeves.Changing the length offers the possibility for distraction during thesurgical procedure in order to adjust the length of the implant 1 to thepatient's requirements.

As also shown in FIGS. 2-4 the implant 1 has a mounting bracket 5″ thatis attached to the upper vertebra 2 and that has a part-spherical anddownwardly concave seat 6 that fits with a complementarily shaped upperend surface 7 of the upper element 4′. The implant 1 is similarlysupported with respect to the lower vertebra 2 shown in the drawing, sothat two substantially identical mounting brackets 5′ and 5″ withrespective seats 6′ and 6″ are provided overall, one of the mountingbrackets 5′ and 5″ being fastened in each of the vertebrae 2, and thelower plate 5″ fitting with a lower end surface 7″ of the lower part 4″.The bearing seats 6′ and 6″ each have a concave part-spherical shape andthe bearing surfaces 7′ and 7″ have a complementary convex shape, andthe centers of curvature of the bearing surfaces 7′ and 7″ lie on theaxis A.

The two parts 4′ and 4″, which have openings 17, form a rotary jointhaving a center of rotation located between the two bearing surfaces 7′and 7″. Due to its design the rotation axis itself is not fixed, so thatthe implant according to the invention permits swiveling in variousdirections. As indicated in FIGS. 3 and 4, it is important that, toallow the natural motion sequence for curvature of the spinal column, itis not necessary for the upper vertebra 4 and the lower vertebra 4 torotate in the same direction.

FIGS. 1-4 drawing also show that on its side contacting the vertebra 2each mounting bracket 5′ and 5″ has a plurality of teeth 9, with pointsedges 8 arrayed in a circle centered on the axis A. These teeth 9 areused to securely anchor the mounting brackets 5′ and 5″ to the vertebrae2, and in particular prevent sliding of the mounting brackets 5′ and 5″with respect to the surface of the vertebrae 2. For securing themounting brackets 5′ and 5″ with respect to the vertebrae 2 lateral tab10 extending parallel to the axis A 10 is also provided extending awayfrom the respective bearing seat 6′ and 6″. A cortical screw 12 engaginginto the respective vertebra 2 passes through an opening 11 in each suchattachment tab 11

The drawing also shows that the two parts 4′ and 4″ are connected to oneanother via an external thread 18 formed on the upper part 4′ and aninternal thread 19 formed on a center part or ring 13 bearing axially onthe top end of the lower part 4″. The upper part parts 4′ engagesaxially inside the lower part 4″ and are displaceable relative to oneanother axially. The parts 4′ and 4″ are locked against relativerotation about the axis A in a simple manner by means of a pin 15projecting diametrally across the upper end of the lower part 4″ andfitting in a pair of diametrally opposite, axially extending, anddownwardly open slots 14 formed in the upper part 4″. Thus, to adjustthe length of the implant 1 the center ring or ring 13 is twisted bymeans of a wrench or key fitted to openings 16 distributed over itscircumference, so that the part 4′ threaded in the center ring 13 isdisplaced axially relative to the other part 4″. After the desireddistraction is achieved, a locking screw is threaded into one of theholes 16, which may be internally threaded, to fix the relative axialposition of the one part 4′ with respect to the other part 4″ by fixingthe ring 13 on the part 4′.

In order to accommodate the brackets 5′ and 5″ to different sizes it ispossible as shown in FIG. 4 for the bracket 5″ to divide the bracketalong a line 20 into two parts that can slide relative to each other andthat can be arrested relative to each other by a screw 21. A dovetailgroove and ridge extending along the line 20 can limit such sliding toone direction. Furthermore a further screw 22 can be provided in the endto relatively shift the parts for accurate adjustment.

1. An implant for insertion into a space between a pair of vertebrae,the implant comprising: two interfitted parts relatively shiftable inthe space along an axis and having outer ends turned axially away fromeach other and each formed with an axially outwardly directedpart-spherical convex surface; means engaged between the parts foraxially shifting them relative to each other and for locking their axialpositions relative to each other; respective L-shaped mounting bracketseach having one leg extending transversely into the space across theaxis and formed with a part-spherical concave seat complementary to andfitting with a respective one of the part outer ends and another legextending outside the space generally axially outward, the bracketsbeing adapted to be fitted to the vertebrae; and means for securing eachof the other legs to the respective vertebra.
 2. The vertebral implantdefined in claim 1 wherein the parts are formed as interfittingcylindrical sleeves centered on the axis.
 3. The vertebral implantdefined in claim 2 wherein the means for axial shifting includes a ringcentered on the axis and threaded to one of the parts and bearingaxially on the other of the parts, whereby rotation of the ring in onedirection forces the two parts axially apart.
 4. The vertebral implantdefined in claim 3, further comprising formations preventing relativerotation of the parts about the axis.
 5. The vertebral implant definedin claim 4 wherein the formations include an axially extending slotformed on one of the parts and a pin fixed in the other part andextending radially into the slot.
 6. The vertebral implant defined inclaim 5 wherein the pin extends diametrally across the other part andthe one part is formed with two diametrally opposite such slots throughwhich the pin passes.
 7. The vertebral implant defined in claim 1wherein the two brackets are substantially identical.
 8. The vertebralimplant defined in claim l wherein the end surfaces of the parts havecenters of curvature lying on the axis.
 9. The vertebral implant definedin claim 1 wherein each of the one legs has a plurality of teeth engagedin the respective vertebra.
 10. The vertebral implant defined in claim 1wherein the means for securing includes respective screws engagedthrough the other legs transversely of the axis with the respectivevertebrae.
 11. The vertebral implant defined in claim 1 wherein each ofthe parts is formed as a sleeve centered on the axis and formed with anarray of radially throughgoing holes.
 12. The vertebral implant definedin claim 1 wherein each of the part end surfaces can swivel in therespective seat.